Some thoughts that have crossed my mind:
- Duratec 2.3L Gasoline Direct Injection turbo (From Mazdaspeed6/3), the bellhousing bolt pattern matches the RWD Vulcan's and standalone EMS are available for non-factory applications. Motor mounts would probably be the most difficult thing and at that, that is not very difficult at all. However they are still pricey, expect to spend $3,000 for a junkyard engine.
- 4.6L V8, find an early 90s Lincoln in the salvage yards, the bottom end is the same as the 96-01 Mustang/F-150 engines, but they are MUCH cheaper. Grab some PI heads off a 2002+ 4.6L and a Dorman factory replacement intake manifold with the aluminum water crossover and you have a nicely powered V8 with a huge aftermarket for even more power.
- Duratec 3.0L V6, these are tough engines, Noble/Rossion/AER produces twin turbo Duratecs that make up to 970 BHP on an engine dyno and the funny thing is, the 400-ish BHP Noble M12s are running nearly all factory Ford parts. The crank is stock, the rods are stock, the cams are from a European Ford Mondeo ST220, the heads are stock and the pistons are the only thing from the aftermarket to lower the compression ratio. Lincoln LS and Jaguar engines are already setup for RWD (Water pump got moved up to the front of the block) and as I mentioned earlier, the Duratec bellhousing bolt pattern matches the Vulcan's. Plus it has variable valve timing, Megasquirt can control this (A few Lotus Espirit guys have dropped the VVT Duratec in their cars with MSII controlling everything), but there are full tuning suites coming if you just want to use the factory Duratec ECU. The 2009/2010 Ford Escape ditched the electronically controlled VVT actuators for a full mechanical system.